Investigations
New spin on salt
Chicago scientist Thomas Rosenbaum demonstrates
entanglement, a phenomenon that leaves classical physics out in
the cold.
Physicist Thomas Rosenbaum has long sensed something
strange about a salt called lithium holmium yttrium tetrafluoride
(lithium holmium fluoride for short). His lab has been experimenting
with the rose-hued crystals for 15 years, and no matter how low
the researchers drop the temperature—even to within a few
thousandths of a degree of absolute zero, or –459°F, theoretically
the temperature at which all molecular motion ceases—the salt
won’t freeze. “That’s a very peculiar behavior.
There must be some hidden variable to explain it,” says Rosenbaum,
the John T. Wilson distinguished service professor in physics, the
James Franck Institute, and the College—and also the University’s
vice president for research and for Argonne National Laboratory.
Now his team has finally solved the mystery: the material’s
atoms are “entangled,” he and Sayantani Ghosh, SM’01,
PhD’03, reported in the September 2003 Nature.
Dan Dry |
Thomas Rosenbaum checks a freezer that cools samples
to not-quite absolute zero.
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When particles are quantum mechanically entangled,
they have a bond that supercedes how each would act on its own.
According to the rules of classical Newtonian physics, as the temperature
approaches absolute zero, no matter what the material, individual
particles and atoms freeze. Rosenbaum and Ghosh demonstrated that
entanglement can break this rule. The atoms in their salt are connected
by electrons whose spins—or magnetic orientations—are
entangled in pairs. Those magnetic pairs obey a higher power—their
innate bond with one another—rather than classical physics
rules, and so they don’t freeze.
The discovery has created a stir in the physics
world, mainly because it was the first demonstration of entanglement
at the macroscopic level. A centimeter-square chunk of the salt
contains a billion billion pairs of entangled spins, acting collectively
to link a similarly huge number of atoms. Never before had entanglement
been shown on such a gigantic scale. “In the world...around
us nature follows certain rules and forms its own patterns, and
we’re familiar with how those rules work and what patterns
we might see,” Rosenbaum says. “In the microscopic world
those rules and patterns are not so easy to intuit because quantum
mechanics apply, so we have to start small and build up. With this
experiment, we’ve taken our understanding of entanglement
another step up.”
Previously the phenomenon had been demonstrated
at the subatomic level, between two photons, for example, and between
a few atoms. “While extremely elegant,” says Ghosh,
who now conducts postdoctoral research at the University of California,
Santa Barbara, “those experiments required excruciatingly
delicate methods.”
Ghosh and Rosenbaum’s experiment was, in
comparison, relatively simple. Although the researchers knew that
lithium holmium fluoride wouldn’t freeze, they didn’t
realize why until they played with another of the compound’s
quirks: its magnetic susceptibility. The spins of the salt’s
holmium atoms move freely, flipping around like compass needles.
At very low temperatures the spins “calm down,” Rosenbaum
explains, and become susceptible to the pull of an outside magnetic
field.
Rosenbaum and Ghosh sent a charge through a wire
looped around the salt crystal, and the holmium atoms aligned with
the outside force. “They sense that there’s a magnetic
field out there, and they want to follow it,” Rosenbaum says.
The researchers used this quirk to learn more about the salt’s
spins. When left undisturbed, the spins have the bizarre quantum
ability to occupy two states at once, to point both up and down
at the same time, an occurrence called superposition.
Never before had entanglement been
shown on such a large scale. “We’ve taken our
understanding,” Rosenbaum says, “one step up.”
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Exposing the spins to the outside magnetic field,
Rosenbaum explains, “collapses the wave function,” which
is physics-speak for making the spins choose one state or another,
up or down. “I can use the magnetic field to twiddle them
just a little bit and interrogate them: ‘are you an up or
a down?’” In that way, the researchers discovered that
a spin’s orientation “is dominated by what its neighbors
are doing”—at a probability that far exceeds the predictions
of classical physics. “Entanglement,” Rosenbaum says,
“was the only explanation.”
The discovery is a milestone in the quest for
a quantum computer, a tiny device capable of making lightning-fast
computations. The fumbling hands and clunky manufacturing capabilities
of humans can construct something only so small before the very
process of building the thing messes it up. Nanoscientists are encouraged
by Rosenbaum’s finding because they study materials that form
patterns on their own and try to harness that self-assembly. The
spins in the lithium holmium fluoride self-assemble into interconnected
peaks and valleys that resemble sand dunes—or perhaps the
wires on a computer chip.
What makes Rosenbaum’s salt remarkable
is that its entanglement results in a magnetic susceptibility that’s
“robust to disorder,” he explains. “It can still
form patterns and act in concert to overcome the disorder we introduce”—kicking
it with an outside magnetic field, exposing it to absolute zero.
Ghosh puts it another way: “Our experiment says we may not
need to focus solely on making a quantum computer. The basis for
a future machine may already exist in nature.”
This particular material won’t likely be
used as a quantum computer because its entanglement is too sensitive
to temperatures. Even heating the salt to one degree above absolute
zero causes its spins to “become too excited to care what
their neighbors are doing.” The question thus becomes, Rosenbaum
says, “Is this material so peculiar that it’s the only
place we see entanglement, or is the phenomenon more universal?”
He, of course, suspects the latter.—S.A.S.
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